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*args and **kwargs - packing and unpacking arguments

python
Table of contents

Packing and Unpacking arguments

We use two operators * for tuples, lists and ** for dictionaries.

Unpacking arguments

We can use * to unpack the list so that all elements of it can be passed as different parameters.

python
# Function that takes 4 arguments and prints them 
def fun(a, b, c, d): 
    print(a, b, c, d) 
  
my_list = [1, 2, 3, 4] 
  
# Unpacking list into four arguments 
fun(*my_list)
# (1, 2, 3, 4)

For dictionary we will use **

python
# Unpacking of dictionary items using ** 
def fun(a, b, c): 
    print(a, b, c) 
  
# A call with unpacking of dictionary 
d = {'a':2, 'b':4, 'c':10} 
fun(**d) 
# 2 4 10

Packing arguments

When we don’t know how many arguments need to be passed to a python function, we can use * to pack all arguments in a tuple.

python
# This function uses packing to sum 
# unknown number of arguments 
def mySum(*args): 
    sum = 0
    for i in range(0, len(args)): 
        sum = sum + args[i] 
    return sum 
  
print(mySum(1, 2, 3, 4, 5)) 
print(mySum(10, 20)) 
# 15
# 30

For dictionary we will use **

python
# Packing of dictionary items using ** 
def fun(**kwargs): 
    for key in kwargs: 
        print("%s = %s" % (key, kwargs[key])) 
fun(name="geeks", ID="101", language="Python")
# language = Python
# name = geeks
# ID = 101

*args and **kwargs

These utilizes arguments unpacking

*args and **kwargs are mostly used in function definitions. You could have also written *var and **vars. Writing *args and **kwargs is just a convention.

*args and **kwargs allow you to pass a variable number of arguments to a function.

Usage of *args

*args is used to send a non-keyworded variable length argument list to the function.

Here’s an example to help you get a clear idea:

python
def test_var_args(f_arg, *argv):
    print("first normal arg:", f_arg)
    for arg in argv:
        print("another arg through *argv:", arg)
test_var_args('yasoob', 'python', 'eggs', 'test')
# first normal arg: yasoob
# another arg through *argv: python
# another arg through *argv: eggs
# another arg through *argv: test

I hope this cleared away any confusion that you had.

Usage of **kwargs

**kwargs allows you to pass keyworded variable length of arguments to a function. You should use **kwargs if you want to handle named arguments in a function.

Here is an example to get you going with it:

python
def greet_me(**kwargs):
    for key, value in kwargs.items():
        print("{0} = {1}".format(key, value))
greet_me(name="yasoob", age=21)
# name = yasoob
# age = 21

This is just the basics of **kwargs and you can see how useful it is.

Using *args and **kwargs

Now that you have learned what *args and **kwargs are for, you are ready to start writing functions that take a varying number of input arguments.

But what if you want to create a function that takes a changeable number of both positional and named arguments?

In this case, you have to bear in mind that order matters. Just as non-default arguments have to precede default arguments, so *args must come before **kwargs.

To recap, the correct order for your parameters is:

  • Standard arguments
  • *args arguments
  • **kwargs arguments
python
# correct_function_definition.py
def my_function(a, b, *args, **kwargs):
    pass

The most common use case is when making function decorators

Fancy uses of unpacking operator

python
my_list = [1, 2, 3]
print(*my_list)
# 1 2 3
print(*my_list, sep=", ")
# 1, 2, 3

Unpack and assign to variable

python
my_list = [1, 2, 3, 4, 5, 6]
a, *b, c = my_list
print(a)
print(b)
print(c)
# 1
# [2, 3, 4, 5]
# 6

Merging lists

python
my_first_list = [1, 2, 3]
my_second_list = [4, 5, 6]
my_merged_list = [*my_first_list, *my_second_list]
print(my_merged_list)
# [1, 2, 3, 4, 5, 6]

Merging dictionaries

python
my_first_dict = {"A": 1, "B": 2}
my_second_dict = {"C": 3, "D": 4}
my_merged_dict = {**my_first_dict, **my_second_dict}
print(my_merged_dict)
# {'A': 1, 'B': 2, 'C': 3, 'D': 4}

String to list

* operator works on any iterable object, and string is an iterable object

python
a = [*"RealPython"]
print(a)
# ['R', 'e', 'a', 'l', 'P', 'y', 't', 'h', 'o', 'n']

Another, fancy one liner

py
*a, = "RealPython"
print(a)
# ['R', 'e', 'a', 'l', 'P', 'y', 't', 'h', 'o', 'n']

There’s the unpacking operator , followed by a variable, a comma, and an assignment. That’s a lot packed into one line! In fact, this code is no different from the previous example. It just takes the string RealPython and assigns all the items to the new list a, thanks to the unpacking operator .

The comma after the a does the trick. When you use the unpacking operator with variable assignment, Python requires that your resulting variable is either a list or a tuple. With the trailing comma, you have actually defined a tuple with just one named variable a.

While this is a neat trick, many Pythonistas would not consider this code to be very readable. As such, it’s best to use these kinds of constructions sparingly.

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Imran Pollob